To date, organic EL devices including a plurality of emitting layers each of which emits light of a different wavelength are known. Such organic EL devices are also known to provide mixed-color light in which the lights emitted by the emitting layers are mixed together. One of such organic EL devices includes a layered red-emitting layer, green-emitting layer and blue-emitting layer, and provides white light in which emissions from the emitting layers are mixed together.
In recent years, a further progress has been made in the development of phosphorescent materials utilizing the emission from triplet exciton energy, and devices of high luminous efficiency have been realized (e.g., Patent Document 1).
Hence, some expect that devices for mixed-color emission will be obtainable by using phosphorescent materials that provide emission at different wavelengths, in particular, white-emitting devices.
However, a phosphorescent material capable of providing long-wavelength emission of green to red color has been known so far, but no phosphorescent material capable of providing short-wavelength emission (blue emission) at a practical level has been known.
Thus, one possible solution is to obtain short-wavelength emission (blue emission) from fluorescent emission while using a phosphorescent material for long-wavelength emission (green to red emission).
However, while the quantum efficiency of the phosphorescent emission can be enhanced up to be 75% or more or approximated to 100%, the luminous efficiency of fluorescent blue emission is typically low. Thus, balancing of the mixed color (e.g., white balance) is difficult.
One possible approach would be to increase the exciton generation in the fluorescent-emitting layer for enhancing the luminance of the blue emission up to the luminance of the phosphorescent emission. However, such an approach would invite increase in the load applied on the fluorescent-emitting layer, so that the degradation of the materials of the fluorescent-emitting layer would be accelerated. Thus, the device lifetime would be considerably shortened.
In view of such problems, there has been a demand for a mixed-color-emitting device (e.g., white-emitting device) having practically-applicable luminous efficiency and lifetime.    Patent Document 1: US2002/182441